How to return a specific point after an error in 'while' loop

Question

I'm trying to write a program that include a while loop, in this loop I have an error message if something goes wrong. It's kinda like this;

while True:

    questionx = input("....")
    if x =="SomethingWrongabout questionX":
        print ("Something went wrong.")
        continue
    other codes...

    questiony = input("....")
    if y == "SomethingWrongabout questionY":
        print ("Something went wrong.")
        continue

    other codes...

    questionz = input("....")
    if z == "SomethingWrongabout questionZ":
       print ("Something went wrong.")
       continue

    other codes..

The problem is as follows: when an error occurs after questionX, the program goes to beginning. It starts from the beginning, not from y or z. But at x there is no problem, so that, the program should start asking questions from y or z because, the problem occurred at y or z.

How can I make the program start from a specific point, like if there is an error only at yquestion, program must start asking questions from y or if only at z,program must start from z, not beginning-not x.

Should I use more than one while loop for this or is there anything that makes this work only in one loop?

Answer 1

[EDIT from generator to function]

You can try a function:

def check_answer(question, answer):
    while True:
        current_answer = input(question)
        if current_answer == answer:
            break
        print "Something wrong with question {}".format(question)
    return current_answer

answerX = check_answer("Question about X?\n", "TrueX")
answerY = check_answer("Question about Y?\n", "TrueY")
answerZ = check_answer("Question about Z?\n", "TrueZ")

Not sure if you want to keep the values, but if you need to tweak it, this should give you hints.

Results:

Question about X?
"blah"
Something wrong with question Question about X?

Question about X?
"blah"
Something wrong with question Question about X?

Question about X?
"TrueX"
Question about Y?
"TrueY"
Question about Z?
"blah"
Something wrong with question Question about Z?

Question about Z?
"blah"
Something wrong with question Question about Z?

Question about Z?
"TrueZ"

Edit per comment:

def check_answer(question, answers):
    while True:
        current_answer = input(question)
        if current_answer in answers:
            break
        print "Something wrong with question {}".format(question)
    return current_answer

answerX = check_answer("Question about X?\n", ("TrueX", "TrueY")

Answer 2

I think here are two, very simple, elegant solutions.

The idea is that there is a list of questions to ask. Both implementations keep on asking as long as questions remain. One will use the itertools.dropwhile() method to drop elements from the list as long as the answer to the question is correct, the other does something different - see below.

In this example implementation, the magic answer 'foo' is the incorrect answer to any question. You can run this in Python to check that it will restart asking the (remaining) questions at the question where you answered 'foo'.

It should be straightforward to adapt to your situation by modifying the ask_question() function.

import itertools

input = lambda x: raw_input("what is your "+x+"? ")

# returns true or false; wether or not the question was answered 
# correctly
def ask_question(question):
    answer = input(question)
    # could be any test involving answer
    return answer != "foo"

# assume we have a list of questions to ask
questions = [ "age", "height", "dog's name" ]

# keep on looping until there are questions
while questions:
    questions = list(itertools.dropwhile(ask_question, questions))

EDIT So, behind the scenes, there are still two while loops (the takewhile() is a giveaway :-)). With a bit of thinking-out-of-the-box, it can be done without even a single while loop:

Recursion's the word!

def ask_more_questions(question_list):
    # no more questions? then we're done
    if not question_list:
        return
    # ask the first question in the list ...
    if ask_question(question_list[0]):
        # ok, this one was answered fine, continue with the remainder
        ask_more_questions(question_list[1:])
    else:
        # Incorrect answer, try again with the same list of questions
        ask_more_questions(question_list)

which can be compressed to, if you like:

def ask(q_list):
    if qlist:
        ask(q_list[1:]) if ask_question(q_list[0]) else ask(q_list)

Answer 3

You are misunderstanding the way to use continue, continue moves to the next iteration of the loop. To fix this just remove the continues

EDIT BASED ON COMMENTS::

I only use the while True values because I don't know anything else about your system

while True:
    while True:
        questionx = input("....")
        if x =="SomethingWrongabout questionX":
            print ("Something went wrong.")
            continue
        else:
            break;

Making use of break will help you achieve what you want

Answer 4

The problem is going to be solved by multiple while loops. Whether those loops are all on one place, or are factored out into functions/generators/etc., is your choice.

If it were me, I would factor out the question-asking code into a function that takes the question itself, plus the verification code to validate the answer -- the function keeps asking the question until the validation passes:

def ask_question(question, validate):
    while "not valid":
        answer = input(question)
        if validate(answer):
            return answer
        else:
            print(" invalid response, try again")

while True:

    x = ask_question("....", lambda a: a=="SomethingWrongabout questionX")

    ...other codes...

    y = ask_questiony("....", lambda a: a== "SomethingWrongabout questionY")

    ...other codes...

    z = ask_questionz("....", lambda a: a=="SomethingWrongabout questionZ")

Answer 5

Yes, there is no way to return to a prior line in code after execution except via a loop. No way at all.

Python and many modern programming languages work in this way and do not support a "goto" line.

Hence, this is the reason the only way to do it is by some form of multiple while loops to execute a statement repeatedly till your desired outcome was received (Either nested loops, or by pulling out the while loop into a function as suggested by salparadise).

Answer 6

Is it possible to put your code inside a function? Knowing that the questions follow an arbitrary order, you can just make use of try/except blocks if the answers don't meet your criteria, and keep a list of the questions that have already been answered.

Let's say we have a global list:

answered_questions = []

And a helper function to let me check if the question has already been answered based on the previous list's length:

def is_it_answered(index):
    """
    Ckecks whether the question number "index" has already been answered.
    :param index: Number of question inside answered_questions
    :return: True if the question was already asked
    """
    # Checking for the index value to be True may not be necessary, but it's just for safety
    if len(answered_questions) >= index + 1 and answered_questions[index]:
        return True

Now all you have to do inside your main function is put inside each suite the code corresponding to each question. If the answer you don't want is entered, raise an exception, not before doing whatever you want before finishing the logic behind that question.

def ask_questions():

    if not is_it_answered(0):
        try:
            answered_questions.append(True)
            questionx = input("...")

            # Whatever is supposed to make Question X wrong goes here
            if questionx == "not what i want":
                raise Exception

        except Exception:
            print "Something went wrong in question x"
            # do whatever you want to do regarding questionx being wrong
            ask_questions()

        # Rest of Code for Question X if everything goes right

    if not is_it_answered(1):
        try:
            answered_questions.append(True)
            questiony = input("...")

            # Whatever is supposed to make Question Y wrong goes here
            if questiony == "not what i want":
                raise Exception

        except Exception:
            print("Something went wrong")
            # do whatever you want to do regarding questionxy being wrong
            ask_questions()

        # Rest of Code for Question Y if everything goes right

    if not is_it_answered(2):
        try:
            answered_questions.append(True)
            questionz = input("...")

            # Whatever is supposed to make Question Z wrong goes here
            if questionz == "not what i want":
                raise Exception

        except Exception:
            print("Something went wrong")
            ask_questions()

        # Rest of Code for Question Z

        # If this is the last question, you can now call other function or end

if __name__ == "__main__":
    ask_questions()

In this code, typing "not what i want" will raise the Exception, and inside the except block, your function will be called again. Be aware that any code that is not indented inside an if condition will be repeated as many times as questions have been asked, just as a precaution.

Answer 7

Just iterate over the questions using an iterator, don't call next on the iterator until you get the output you require:

questions = iter(("who is foo", "who is bar", "who is foobar"))
def ask(questions):
    quest = next(questions)
    while quest:
        inp = input(quest)
        if inp != "whatever":
            print("some error")
        else:
            print("all good")
            quest = next(quest, "")

If you have questions and answers just zip them together:

def ask(questions, answers):
    zipped = zip(questions,answers) # itertools.izip python2
    quest,ans = next(zipped)
    while quest:
        inp = input(quest)
        if inp != ans:
            print("Wrong")
        else:
            print("all good")
            quest, ans = next(zipped, ("",""))

Answer 8

Set x, y and z to None before entering loop. Then protect each question with if and set the variable in question to None again before continue.

x = y = z = None
while True:

    if x is None:
        questionx = input("....")
        if x =="SomethingWrongabout questionX":
            print ("Something went wrong.")
            x = None
            continue

        other codes...

    if y is None:
        questiony = input("....")
        if y == "SomethingWrongabout questionY":
            print ("Something went wrong.")
            y = None
            continue

        other codes...

    if z is None:
        questionz = input("....")
        if z == "SomethingWrongabout questionZ":
           print ("Something went wrong.")
            z = None
           continue

        other codes..  

Answer 9

The problem is one of program cohesion. If you have particular questions that have specific validations, you should write functions for them..

def getX():
   while True:
      response = input("...")
      if response == "something wrong with x":
         print("Something went wrong with x")
      else:
         return response

def getY():
   ...

Then in your code you just

x = getX()
y = getY()
z = getZ()

Each of these functions can validate the input in different ways. You can also try to generalize them if many of your validations fall into a particular pattern. E.g.

def getInt(name, range_start, range_end):
   prompt = "Enter a number for {} between {} and {}".format(name,
                                                             range_start, 
                                                             range_end)
   while True:
      try:
          response = int(input(prompt))
      raise ValueError:
          print("That's not a number")
          continue
      if response not in range(range_start, range_end+1):
          print(response, 'is not in the range')
      else:
          return response

Answer 10

A simple solution to the problem would be to use a counter variable to address the issue.Something like this:

counter = 0
while True:
    if counter == 0:
        questionx = input("....")
        if x =="SomethingWrongabout questionX":
            print ("Something went wrong.")
            continue
        else:
            counter = counter + 1
         other codes...

    if counter <= 1:
        questiony = input("....")
        if y == "SomethingWrongabout questionY":
            print ("Something went wrong.")
            continue
        else:
            counter = counter + 1
        other codes...

    if counter <= 2:
         questionz = input("....")
         if z == "SomethingWrongabout questionZ":
             print ("Something went wrong.")
             continue
         else:
             counter = counter + 1
        other codes..

The idea is to increment the counter every-time something goes right. After the counter has been incremented it won't execute other conditions and will jump directly to the code block where it went wrong

Answer 11

I'd do it this way:

qa = (
    ('Question X', 'Answer X'),
    ('Question Y', 'Answer Y'),
    ('Question Z', 'Answer Z'),
)

for item in enumerate(qa):
    question = item[1][0]
    answer = item[1][1]
    while True:
        usr = input("What is the answer to %s: " % question)
        if usr == answer:
            break

This results to:

$ python qa.py
What is the answer to Question X: Answer X
What is the answer to Question Y: Answer Y
What is the answer to Question Z: Answer X
What is the answer to Question Z: Answer Z

Process finished with exit code 0